Various methods have conventionally been known for treating or finishing the surface of metal articles, such as chemical coating, plating, etching, etc. Above all chemical conversion treatment for forming some kinds of corrosion resistant films on the surface of steel plates or steel bars by means of chemical reaction or electro-chemical reaction is known, and more particularly phosphating, wherein zinc phosphate, manganese phosphate, ferrous phosphate, etc., are employed, is widely known. And it is widely practiced as a method of surface preparation when steel members are coated in the field of, for example, car assemblying, bridge building, or manufacturing of household electrical appliances.
When practicing such a surface treatment, objects-to-be-treated must be in general contacted with the treating liquid, and either of two ways, a spraying method in which the liquid is sprayed over the surface of the objects-to-be-treated or a dipping method in which the objects are dipped in the liquid, is selected for contacting the objects with the liquid. The former spraying method is unsuitable for treating objects of complicated shape, for example car bodies which have portions difficult to be uniformly treated by spraying, such as a fender, a door, or a constructed part in a box-like shape. The liquid is liable not to sufficiently reach the inside surface of complicated corner, sometimes leaving untreated portions or unsatisfactorily treated portions. The latter dipping method is much better than the former in respect of the liquid reaching many quarters of the objects-to-be-treated, but it still leaves something to be desired because the sludge by products are deposited on the bottom of the treating vessel during dipping of the objects-to-be-treated into the liquid, so as to cause some troubles in a continuous surface treatment.
In a chemical conversion process as one of the surface treatments, more specifically speaking, chemical reaction taking place between a steel plate as an object-to-be-treated and a treating liquid containing phosphate which produces crystal coating on the surface of the steel plate. In the course of this chemical reaction, however, by-products called sludge are produced. This chemical conversion sludge created in the course of a secondary reaction did not give rise to serious trouble in the conventional spray method as shown in FIG. 1 wherein phosphate conversion coating was made on the surface of the object W.sub.1 by means of the spraying S, because the forced pressure of the spraying S naturally prevented depositing of the sludge onto the coated surface of the object W.sub.1. The sludge has however recently become a serious factor for deteriorating the quality of the articles since the surface treatment had been gradually changed to the full dipping treatment method, wherein the object is completely dipped in the treating liquid. In this full dipping method, the sludge is liable to float in suspension in the treating liquid. This kind of chemical conversion sludge not only floats in a treating vessel containing the liquid but is also stirred up from the bottom of the vessel on which it has once been deposited due to the movement of the object-to-be-treated in the vessel. The sludge possibly stuck to the crystal coating or contained in the formed coating deteriorates the corrosion resistance of the coating or adversely affects the resulting electro-depositing process. It is therefore highly desirable to discharge or exhaust such harmful problem-arising sludge out of the vessel.
Conventionally, leading of the liquid out of the system or vessel, by recycling the liquid with equipment shown in FIG. 2, was practiced for removing the sludge. In this equipment, roughly speaking, the liquid containing high density sludge collected in a hopper 4 disposed at the bottom of a treating vessel 2 is led to a sludge removing apparatus 8 through a conduit 6 by the action of a pump P.sub.1. The liquid is separated from the sludge by slits 10 disposed in the sludge removing apparatus 8 and is recycled to an overflow tank 14 (recovering tank) of the treating vessel 2 through a conduit 12 by the action of a pump P.sub.2. It is again recycled, together with the liquid which is overflowed from the treating vessel 2 into the overflow tank 14, to the treating vessel 2 through a conduit 16 by the action of a pump P.sub.3 for being circulated within the treating vessel 2. Numeral 18 designates the surface of the liquid.
The sludge removing apparatus 8 disposed outside the vessel is apt to occupy a substantial space and impose a substantial cost. It is naturally accompanied by some other problems such as complicated piping for recycling, increasing of the recycled amount of the liquid, etc., and a still unsolved problem is stirring up of the deposited sludge on the vessel bottom.